These aren’t exactly free wargames rules, but if you already own a set of Warhammer 40K, this is worth checking out. Jervis Johnson has provided this page of colonial amendments to Warhammer 40K. These also would be a good way to introduce fantasy and science fiction wargamers to historical miniatures games.
The Miniature Wargaming Editor
Free English Civil War Flags
This 28mm Life offers a large collection of free print-and-play flags from the English Civil War.
Nelson’s Wars Napoleonic Naval Rules
Nelson’s Wars is a set of free wargames rules for naval games in the age of Napoleon.
Painting Tiger Stripe Camouflage
Here’s a nice tutorial on painting tiger stripe camouflage.
Imperial Guards Vs Zombies Rules
Imperial Guard vs Zombies is a set of modifications for Warhammer 40K